Toddler Rule #10,299
If you don’t want to smile for a picture — you don’t have to.
And, (rule extension 1a) in the event the photographer (especially in the case of a parent photographer) insists on taking your picture, you may display the emotion (happy, sad, grumpy, sassy, etc.) of your choosing.
Regardless of the expression, they (usually parents) will probably end up liking the picture anyway.
They’ll think they’ve won the battle, but points really go to you my friend. Points go to you.